1. Field of the Invention
The present invention generally relates to a scramble code allocation method in CDMA (Code Division Multiple Access) cellular systems, and especially relates to a scramble code allocation method applied to a radio base station at a new installation, and while in service.
2. Description of the Related Art
In the CDMA cellular systems, methods for assigning a scramble code to each radio base station are disclosed as follows.
According to a first conventional method, a scramble code of a radio base station is identified by detecting a time difference (gap) between the time when the scramble code is transmitted by a transmitter and the time when the scramble code is received by a receiver. The time difference is determined in units of chips. This is realized by using a standard time common to all apparatuses in a mobile communications system, employing the GPS (Global Positioning System).
Specifically, a BTS control unit (Base Transceiver Station Controller) identifies a first radio base station. Then, the first radio base station is made to transmit a pilot channel using a known scramble code. Then, other radio base stations, called the second through Nth radio base stations, receive the pilot channel from the first radio base station, detect the gap of the scramble code in units of chips, and store the gap values. When all the second through Nth radio base stations finish storing the gap values, the first radio base station stops transmission. Then, the second radio base station transmits a pilot channel using a known scramble code, and other radio base stations detect and store the chip gap values. This operation is repeated until all the N radio base stations finish the pilot channel transmission. Then, distances between each of the radio base stations is computed by a predetermined method based on the chip gap values stored in the predetermined storing place. After computing the distances between each of the radio base stations, a scramble code is assigned to each radio base station based on the distances (see Patent Reference 1).
According to a second conventional method, a radio base station has a search function that searches for sync channels transmitted by other radio base stations (radio base stations of other cells and the like), and autonomous synchronization between the radio base stations, assignment of a scramble code, and assignment of a frequency are carried out.
That is, the radio base station includes a scramble code identifying unit that identifies scramble codes of other radio base stations based on a de-scrambled signal and an output result of an A/D converter, and outputs the identified result to a code allocating unit. The code allocating unit outputs an optimal scramble code to the radio base station based on the identified scramble codes of other radio base stations, and the code allocating unit assigns the optimal scramble code to the radio base station based on the identified scramble codes (see Patent Reference 2).
[Patent reference 1] JPA 2000-32531
[Patent reference 2] JPA 2002-218528